In the field of medicine, radiation may be used for diagnostic, therapeutic and palliative purposes. Therapeutic use of radiation such as x-rays and γ-rays typically involves-using these rays to eradicate malignant cells. Conventional radiation treatment systems used for medical treatment, such as the linear accelerators that produce high-energy x-rays, utilize a remote radiation source external to the targeted tissue. A beam of radiation is directed at the target area, for example a malignant tumor inside the body of a patient. The x-rays penetrate the patient's body tissue and deliver x-ray radiation to the cancer cells, usually seated deep inside the body. This type of treatment is referred to as teletherapy because the radiation source is located at some distance from the target. This treatment suffers from the disadvantage that tissue disposed between the radiation source and the target is exposed to radiation. To reach the cancer cells, the x-rays from an external radiation source must usually penetrate through normal surrounding tissues. Non-cancerous tissues and organs are thus also damaged by the penetrating x-ray radiation.
Brachytherapy, on the other hand, is a form of treatment in which the source of radiation is located close to, or in some cases within, the area receiving treatment. Brachytherapy, a word derived from the ancient Greek word for close (“brachy”), offers a significant advantage over teletherapy, because the radiation is applied primarily to treat only a predefined tissue volume, without significantly affecting the tissue adjacent to the treated volume. The term brachytherapy is commonly used to describe the use of “seeds,” i.e. encapsulated radioactive isotopes, which can be placed directly within or adjacent the target tissue to be treated. Handling and disposal of such radioisotopes, however, may impose considerable hazards to both the handling personnel and the environment. Also, introduction of the radioisotopes requires invasive procedures which have potential side-effects, such as the possibility of infection. Moreover, there is no ability to provide selective control of time dosage or radiation intensity.
The term “x-ray brachytherapy” is defined for purposes of this application as x-ray radiation treatment in which the x-ray source is located close to or within the area receiving treatment. An x-ray brachytherapy system, which utilizes a miniaturized low power radiation source that can be inserted into, and activated from within, a patient's body is disclosed in U.S. Pat. No. 5,153,900 issued to Nomikos et al., U.S. Pat. No. 5,369,679 to Sliski et al., U.S. Pat. No. 5,422,926 to Smith et al., and U.S. Pat. No. 5,428,658 to Oettinger et al., all owned by the assignee-of the present application, all of which are hereby incorporated by reference. The x-ray brachytherapy systems disclosed in the above-referenced patents include miniaturized, insertable x-ray probes that are capable of controllably producing and delivering low power x-ray radiation, while positioned within or in proximity to a predetermined region to be irradiated. In this way, x-ray radiation need not pass through the patient's-skin, bone, or other tissue prior to reaching the target tissue. The probe may be fully or partially implanted into, or surface-mounted onto a desired area, within a treatment region of a patient. The insertable probe emits low power x-rays from a nominal, or effective “point” source located within or adjacent to the desired region to be irradiated, so that substantially only the desired region is irradiated, while irradiation of other regions are minimized. X-ray brachytherapy offers the advantages of brachytherapy, while avoiding the use and handling of radioisotopes. Also, x-ray brachytherapy allows the operator to control over time the dosage of the delivered x-ray radiation.
X-ray brachytherapy treatment generally involves positioning the insertable probe into or adjacent to the tumor, or into the site where the tumor or apportion of the tumor was removed, to treat the tissue adjacent the site with a local boost of radiation. X-ray probes of the type generally disclosed in U.S. Pat. No. 5,153,900 include a housing, and a hollow, tubular probe or catheter extending from the housing along an axis, and having an x-ray emitting target element at its distal end. The probe may enclose an electron source, such as a thermionic cathode. In one form of a thermionic cathode, a filament is resistively heated with a current. This in turn heats the cathode so that electrons are generated by thermionic emission.
In another form of an x-ray brachytherapy device, as disclosed in U.S. Pat. No. 5,428,658, an x-ray probe may include a flexible probe, such as a flexible fiber optical cable enclosed within a metallic sheath. The x-ray probe may also include a substantially rigid, evacuated capsule that is coupled to a distal end of the flexible probe. The capsule encloses an optically activated electron source, such as a photocathode, and an x-ray emissive target element. In a photocathode configuration, a photoemissive substance is irradiated by a LED or a laser source, causing the generation of free electrons. Typically, the flexible fiber optical cable couples light from a laser source or a LED to the photocathode.
U.S. patent application Ser. No. 09/884,561 and hereby incorporated by reference)(hereinafter the “'561” application) discloses an optically driven (for example, laser driven) x-ray source using a reduced-power, increased efficiency electron source, which generates electrons with minimal heat loss. The '561 application discloses the use of laser energy to heat an electron emissive surface of a thermionic emitter, instead of using an electric current to ohmically heat an electron emissive surface of a thermionic emitter. With the optically driven thermionic emitter, electrons can be produced in a quantity sufficient to produce the electron current necessary for generating therapeutic radiation at the target, while significantly reducing the requisite power requirements.
Even though the above-discussed miniature radiation sources can generate x-rays local to the target tissue, it is difficult to provide a uniform, or other desired, dose of radiation to an irregularly shaped target tissue, using these radiation sources. In one form, these radiation sources generally act as point sources of therapeutic radiation. The intensity of the radiation from a point source decreases uniformly with approximately the square of the distance (R) from the source (i.e., 1/R2). Since body cavities, or the beds of resected tumors, are not generally spherically symmetrical, a point source within a body cavity or central to the resected tumor bed will not deliver a uniform dose of radiation to the tissue lining of the cavity or bed. Similarly, for a non-spherical tumor, a point source at the tumor center will not deliver radiation with an isodose contour matching the peripheral surface of the tumor. U.S. Pat. No. 5,422,678 to Dinsmore et al. (the “'678 patent”) discloses an x-ray source which allows the x-ray target to emit x-rays in a predetermined spectral range, by providing a beam steering assembly that controls the focus and deflection of the electron beam incident on the target. The beam steering assembly includes means for sensing the deflection of the electron beam, by monitoring the back-scattered x-rays, i.e. the x-rays emitted from the target in a backward direction along the path of the electron beam. A feedback signal is generated in response to the sensed deflection. The feedback signal is provided to an electron beam deflection controller, which controls the electron beam in response to the feedback signal.
In U.S. Pat. No. 5,422,926 to Smith et al. (the “'926 patent”)(commonly owned by the assignee of the present application, and hereby incorporated by reference), an x-ray source is disclosed that is adapted for irradiating a volume in accordance with a predetermined dose distribution. In the '926 patent, a variable thickness x-ray shield is disclosed, which allows the irradiation of a preselected volume, defined by a set of isodose contours. This type of shielding around the x-ray target, or at the emission site, enables control of the energy and spatial profile of the x-ray emission, to match the preselected distribution of radiation throughout the deisred region.
The treatment regions within a patient's anatomical structure, however, are usually not adapted for uniform or predefined patterns of irradiation, because the organs or body cavities generally have arbitrary and irregular shapes and geometries. Cancerous tumors are also usually shaped irregularly, and are distributed randomly across a given anatomical region. A spherically isotropic spatial distribution of therapeutic radiation may not be suitable in many cases, for such arbitrarily and irregularly shaped treatment regions. The areas of a patient's body requiring treatment may be characterized by twists and bends. In some cases, the geometry of the target region may not be fixed, in the bladder for example, which has a flexible inner wall without a well-defined shape. Also, some treatment procedures may require delivery of localized radiation to portions of the human body that are not easily accessible.
Accordingly, there is a need for a system that permits the surgeon or technician to monitor in real time the delivered dose of therapeutic radiation during the treatment procedure. Such a system would enable the surgeon to regulate the generation and delivery of the therapeutic radiation, in response to the monitored dosage, thereby more accurately deliver therapeutic radiation to a treatment region, as compared to a system in which radiation is delivered according to a pre-planned radiation treatment profile.